New Measurement of the 2S Hyperfine Interval in Atomic Hydrogen

An optical measurement of the 2S hyperfine interval in atomic hydrogen using two-photon spectroscopy of the 1S-2S transition gives a value of 177 556 834.3(6.7) Hz. The uncertainty is 2.4 times less than achieved by our group in 2003 and more than 4 times less than for any independent radio-frequency measurement. The specific combination of the 1S and 2S hyperfine intervals predicted by QED theory $D_{21}=48 953(3)$ Hz is in perfect agreement with the value $48 923(54)$ obtained from this experiment.